The present invention relates in general to integrated circuits and, more particularly, to integrated power factor correction circuits.
Integrated power factor correction (PFC) circuits are switching circuits that ensure that an alternating current (AC) line is loaded with an in-phase, substantially sinusoidal current. Without PFC correction, many electrical systems draw current from the AC line voltage only near the peak voltage levels of the AC line. The aggregate effect of loading the AC line with large currents at the voltage peaks and zero current at other times is to produce distortion of the AC line voltage. In addition, systems without PFC can cause high neutral currents to flow in three-phase distribution networks. To avoid these problems so that electrical devices function properly, the distribution networks of regional utility companies must be oversized, which necessitates a large capital investment. Several governments are mandating that PFC be incorporated in the power supplies used in some or all electrical devices.
PFC circuits typically switch current through an inductor or coil from the AC line at a frequency much higher than the frequency of the AC line to magnetize or charge a coil. For example, systems may use a switching frequency of at least one hundred kilohertz when the AC line frequency is fifty hertz. The energy stored in the coil is discharged into a capacitor to generate an intermediate PFC direct current (DC) supply voltage to power an electrical device or system. To attain a substantially sinusoidal AC current, the average value over a switching period of the current switched through the coil is made proportional to the current voltage on the AC line. The result is a high effective power factor for the AC line.
Most previous PFC circuits operate in a free-running mode in which a current is switched to charge the coil as soon as the coil current stored in the previous cycle has been discharged across the capacitor. As a result, the previous PFC circuits switch at a frequency that varies with the current AC line voltage as well as the system's load current. Such variable switching frequencies are difficult to filter out in order to suppress or remove electromagnetic interference generated by the switched coil currents. Such filtering requires complex filters which dissipate power and substantially increase the manufacturing cost of a system.
Hence, there is a need for a PFC circuit and method of correcting the power factor that operates at a fixed or nearly fixed frequency in order to reduce a system's electromagnetic interference while maintaining a low fabrication cost of the PFC circuit.